Many industrial processes that involve production or treating mineral ores in order to extract mineral values will result in waste material. For example, mineral ores are ground and slurred in water to allow separation and recovery of desired components. The waste material may comprise an aqueous slurry of the residual components, which may comprise particulate mineral material such as clays, sand, grit, metal oxides, etc. These waste materials, often referred to as “tailings” are then typically subjected to dewatering processes. For example, the tailings may be deposited in pits or ponds, sometimes called tailings ponds, where solids are expected to settle and allow recovery of the supernatant water, and ultimate consolidation of the remaining mineral solids. Coal, copper, and gold mining are but a few of the mining processes that employ this technology.
Bituminous sands, also referred to as oil sands, are a type of petroleum deposit. Oil sands typically contain naturally occurring mixtures of sand, clay, water, and a dense, extremely viscous form of petroleum technically referred to as bitumen, or colloquially “tar” due to their similar appearance, odor, and color. Oil sands may be found in large quantities in many countries throughout the world, most abundantly in Canada and Venezuela. Oil sands reserves are an important part of the world's oil reserves, particularly as higher oil prices and new technology enable oil sands reserves to be profitably extracted and upgraded to usable products. Oil sands are often referred to as unconventional oil or crude bitumen, in order to distinguish the bitumen extracted from oil sands from the free-flowing hydrocarbon mixtures known as crude oil traditionally produced from oil wells.
Conventional crude oil may be extracted from the ground by drilling oil wells into a petroleum reservoir and allowing oil to flow into them under natural reservoir pressure, although artificial lift and techniques such as water flooding and gas injection may be required to maintain production as reservoir pressure drops toward the end of a field's life. Since extra-heavy oil and bitumen flow very slowly, if at all, towards producing wells under normal reservoir conditions, the sands may be extracted by strip mining or the oil made to flow into wells by in situ techniques that reduce the viscosity, such as by injecting steam, solvents, and/or hot air into the sands. These processes may use more water and may require larger amounts of energy than conventional oil extraction, although many conventional oil fields also typically require large amounts of water and energy to achieve good rates of production.
Water-based oil sand extraction processes generally include ore preparation, extraction and tailings treatment stages, wherein a large volume of solids-laden aqueous tailings may generally be produced.
These tailings may generally be referred to as oil sands process tailings, or oil sands tailings. These tailings may require solid-liquid separation in order to efficiently recycle the water and reduce the volume of the tailings. In many processes, these oil sands tailings are pumped to large settling ponds or tailings ponds.
In tailings ponds, the process water, unrecovered hydrocarbons and minerals generally settle naturally to form different strata. The upper stratum is usually primarily water that may be recycled as process water to the extraction process. The lower stratum generally contains the heaviest materials, mostly sand, which settle to the bottom of the pond. The middle stratum, often referred to as “mature fine tailings” (“MFT”), generally includes water and hydrophilic and biwetted ultrafine solids, mainly clays and other charged silicates and metal oxides, that tend to form stable colloids in water and exhibit a very slow settling and dewatering behavior, resulting in tailing ponds that may take several years to manage.
The composition of mature fine tailings tends to be highly variable. Near the top of the stratum the mineral content may be about 10% by weight and over time may consolidate and comprise up to 50% by weight of the materials contained at the bottom of the stratum. Overall, mature fine tailings may have an average mineral content of about 30% by weight. While fines may generally be the dominant particle size fraction in the mineral content, the sand content may be 15% by weight of the solids and the clay content may be up to 100% by weight of the solids, reflecting the oil sand ore and extraction process. Additional variation may result from the residual hydrocarbon which may be dispersed in the mineral or may segregate into mat layers of hydrocarbon. The mature fine tailings in a pond may not only contain a wide variation of compositions distributed from top to bottom of the pond, but also may contain pockets of different compositions at random locations throughout the pond. Additionally, mature fine tailings generally behave as a fluid-like colloidal material.
The slow settling of fine (<45 μm) and ultrafine clays as well as the large demand of water during oil sand extraction process have promoted research and development of new technologies to modify the water release and to improve settling characteristics of tailings streams. In treatment processes for dewatering oil sands tailings, clays and ultra-fine solids (<2 μm) are often challenging to capture and in many instances, may remain suspended in the treated water which will be recycled back to the extraction process. These solids may be detrimental to bitumen recovery, and as such, maximizing separation of the fines from the water during tailings treatment is of general importance. As such, improving the treatment of tailings is of great interest.
In some instances, treatment of mineral or oil sands tailings streams may generally comprise the use of flocculants. Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, thereby forming a floc. For example, flocculants are used in water treatment processes to improve the sedimentation or filterability of small particles. Flocculants that have been used in treatments for dewatering mineral tailings and oil sands tailings include polyacrylamide polymer flocculants. Due to the volume of polyacrylamide consumed for mineral or oil sands tailings, dry polyacrylamide (DPAM) is commonly used instead of solution or emulsion polymers. DPAMs typically have standard viscosities (SV) in the range of 3.5-6.5 cP. In mineral or oil sands tailings applications, it has been found that lower molecular weight (MW) products may have the potential to produce flocs with better dewaterability. While higher molecular weight products can provide flocculation, they can be more difficult to mix into the tailings and have a greater tendency to hold water. However, it has been observed that manufacturing low molecular weight DPAMs presents operational challenges because the polymerized gel may be very soft and tacky, which complicates the grinding and drying steps after polymerization.
Optimizing a polymer flocculant commonly involves adjusting the formulation at the point of production to create a polymer flocculant with desired properties, e.g., charge density, molecular weight, conformation, etc. While in theory this optimization can be used to formulate most effective polymer flocculants, in practice polymer production processes may have certain operational constrictions, such as the inability to grind and dry products with low molecular weight, that may limit one's ability to successfully manufacture a desired polymer.
Mineral tailings and oil sands tailings have a range of density and clay content, that can vary over time, and/or by location or other conditions. This variability may present challenges to obtain consistent chemical treatment results. An “optimal” chemical treatment may be formulated, to handle tailings having a specific subset of these properties. Small changes in the properties of the tailings substrate can change (e.g. reduce) the effectiveness of the chemical treatment. One way of overcoming this is to dilute the tailings stream to maintain specific clay to water ratios. Another option is to have multiple flocculants available for different tailings compositions. Neither of these options is ideal.